Saccharomyces

SACCHAROMYCES CEREVISIAE

 

(Dr. Girish Chandra)

 

            The word "Saccharomyces" derives from Greek term that means "sugar mold". "cerevisiae" comes from Latin word meaning "of beer". Saccharomyces cerevisiae is commonly known as "bakers yeast" or "brewers yeast". The yeast ferments sugars present in the flour or added to the dough, giving off carbon dioxide and alcohol (ethanol) to gain energy from the breakdown of carbohydrates. The CO2 is trapped as tiny bubbles in the dough, which rises. Most modern brewers use highly cultured isolates, e.g. Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen.

           

            Saccharomyces belongs to kingdom Fungi because it has a cell wall made of chitin, it has no peptidoglycan in its cell walls, and its lipids are esters. It also uses DNA template for protein synthesis and it has larger ribosomes. It is considered yeast because it is a unicellular organism so it can not form a fruiting body like other fungi do.

 

BIOLOGY

 

            Saccharomyces cerevisiae has both asexual and sexual reproduction. There are two forms in which yeast cells can survive and grow, haploid and diploid. The haploid cells undergo a simple lifecycle of mitosis and growth. The diploid cells similarly undergo a simple lifecycle of mitosis and growth, but under conditions of stress can undergo sporulation, producing a variety of haploid spores, which can go on to conjugate to produce diploid stages again. These diploid cells can go through mitosis, which they call budding or they can undergo meiosis and from an ascus which will split into four ascospores. These haploids can then germinate and become haploid yeast again (Madigan, 457).

 

 

SPECIAL FEATURES

 

            Saccharomyces cerevisiae is a widely used model organism in science, and therefore also is one of the most studied, along with E. coli. S. cerevisiae has obtained this important position because of its established use in industry for making beer, bread and wine and in ethanol production. It is also easy to manipulate and culture in the lab. Additionally, yeasts are comparatively similar in structure to human cells, both being eukaryotic. The availability of the S. cerevisiae genome sequence and the knowledge of complete set of deletion mutants have further enhanced its utility as a model for understanding the regulation of eukaryotic cells.

 

            Saccharomyces cerevisiae has been a very important genetic tool. It has been used in genetic studies for many decades. Since it is very small and unicellular, large numbers of the yeast can be grown in culture in a very small space, in much the same way that bacteria can be grown. However, yeast has the advantage of being an eukaryotic organism, so that the results of genetic studies with yeast are more easily applicable to human genetics.

 

            Saccharomyces cerevisiae reproduces abundantly and quickly, producing large number of haploid cells, which can mate with an appropriate strain, undergoing karyogamy and growing as a diploid organism. The diploids can undergo meiosis to form ascospores, which are a recombinant haploid progeny unlike either parent. Mitosis and meiosis can be more easily studied in these organisms.

 

           

CONTRIBUTIONS TO RESEARCH

 

            Lee Hartwell, from the Fred Hutchison Cancer Research Center in Seattle, won the Nobel Prize in Medicine in 2001 for his pioneering work on the mitosis genes in S. cerevisiae. He shared the prize with R. Timothy Hunt and Paul M. Nurse of the Imperial Cancer Research Institute in London, who worked on another species of yeast, Schizosaccharomyces pombe. The genes they discovered and characterized in the yeast as a model organism have led to some important discoveries in fighting cancer in humans.

 

            Approaches have been developed by yeast scientists, which can be applied in many different fields of biological and medicinal science. These include Yeast hybrids for studying protein interactions, synthetic genetic array analysis for studying gene interaction, and tetrad analysis. Many proteins important in human biology were first discovered by studying their homologs in yeast, for instance, cell cycle proteins, signaling proteins, and protein-processing enzymes. The petite mutation in S. cerevisiae is of particular interest in genetic studies.

 

            Saccharomyces cerevisiae is one of the most important fungi in the history of the world. This yeast is responsible for the production of ethanol in alcoholic drinks. There are two ways Saccharomyces cerevisiae breaks down glucose. One way is through aerobic respiration in the presence of oxygen but when oxygen is not available, the yeast will grow through anaerobic fermentation to gain two ATPs and produce two by products, namely, carbon dioxide and ethanol. So if this yeast is grown in a container lacking oxygen it will produce ethanol. Both of these processes use haploid forms of the yeast for this process. In industry they isolate one strain, either a or ?, of the haploid form to keep them from undergoing mating. In baker’s yeast there is a strain where the production of carbon dioxide is more prevalent than ethanol.

 

            One important use of this yeast is that “live yeast supplementation to early lactating dairy and goats significantly increased milk production”.

 

            Saccharomyces cerevisiae, which can proliferate in both haploid and diploid stages, has been used extensively in aging research. The budding yeast divides asymmetrically to form a mother cell and a bud. Two major approaches, study of life span of buds and the stationary phase have been used to determine senescence and life span in yeast.

 

            The demand for suitable genetically modified (GM) S. cerevisiae strains for producing biofuel, bakery products and beverages or for the production of biotechnological products (e.g. enzymes, pharmaceutical products) is continuously growing. Numerous specialized S. cerevisiae wine producing strains were obtained in recent years that possess a wide range of properties, capable of satisfying the demands of modern winemaking industry.

 

            The unlocking of transcriptome, proteome and metabolome complexities will contribute decisively to the knowledge of genetic make-up of commercial yeast strains and will influence wine strain improvement via genetic engineering.

 

            Yeast extract hydrolysate from Saccharomyces cerevisiae helps to prevent bacterial and fungal diseases in treated plants. It appears to act by enhancing the plant’s natural defense mechanisms. The active ingredient is approved for use on all food crops, as well as on turf and ornamental plants. Yeast extract is a common food flavoring and has a long history of use as a plant fertilizer. No risks to human health or to environment are known to occur by the use of yeast extract hydrolysate.